The present disclosure relates to an image forming apparatus including a drive roller configured to convey a printing medium.
Conventional electrophotographic image forming apparatuses have the function of detecting any occurrence of a jam (a print sheet conveyance failure or a paper jam) in a conveyance path provided within the apparatuses, and provide a notification of the occurrence of a jam. Whether or not a jam has occurred can be determined based on a sensor signal from a sheet sensor for detecting a print sheet being conveyed along the conveyance path. Specifically, it is determined that a jam has occurred if the sheet sensor is continuously detecting a print sheet for a predetermined time or longer. If it is determined that a jam has occurred, the heating operation of a heater or the rotating operation of a heating roller of a fixing device is subjected to emergency stoppage.
In addition, a conventional image forming apparatus has a maintenance opening formed in its housing to allow easy removal of a print sheet jammed in the conveyance path, and a cover is attached to the opening. For an image forming apparatus of this type, it is hazardous if the printing operation is unintentionally performed when the operator is removing a jammed print sheet by opening the cover. Furthermore, there is the possibility that the apparatus may fail when the printing operation is performed with the cover being opened. For this reason, the conventional image forming apparatus is provided with a cover opening/closing detection sensor serving as an interlock switch for detecting opening/closing of the cover. Thereby, the heater or the heating roller of the fixing device is stopped when the cover is open even if a sheet removal operation (jam processing operation) for removing the print sheet has been completed, thus ensuring the safety of the operator and achieving a smooth jam removal operation. Note that if the sheet sensor is no more detecting a print sheet after completion of the sheet removal operation and it is determined that the cover is closed, the heater, the heating roller, and the like of the fixing device are brought into a drivable state, thus enabling the image forming operation.
Here, a jam of the print sheet does not necessarily occur at a position that can be detected by the sheet sensor. For example, the sheet sensor cannot detect such a jam in which a print sheet is wound around the heating roller of the fixing device or a conveyance roller provided on the conveyance path. In addition, a jam may occur in an area where the sheet sensor is not provided. In this case, the occurrence of a jam cannot be directly detected by the sheet sensor, and it is determined that a jam has occurred in the conveyance path if the sheet sensor does not detect a print sheet after an elapse of a predetermined time since an instruction to convey a sheet has been issued. In this case as well, the heater or the heating roller of the fixing device will be stopped for safety. However, if a jammed print sheet cannot be directly detected by the sheet sensor, the image forming apparatus estimates that the sheet removal operation has been performed and the print sheet is removed when the cover is opened and then closed again, regardless of whether the print sheet is actually removed or not. Accordingly, the image forming operation is returned to a state in which it can perform an image forming operation. In this case, performing the image forming operation by driving the heater or the heating roller of the fixing device leads to excessive heating of the jammed print sheet or application of an additional conveyance force to the jammed print sheet, resulting in a failure of the image forming apparatus.
If, on the other hand, the operator pulls out a print sheet jammed in the fixing device during the sheet removal operation in the image forming apparatus, the heating roller is rotated by the frictional force between itself and the pulled-out print sheet. It is known that the output shaft of a rotary drive source, such as a motor, that supplies a driving force to the heating roller is also rotated at this time, and a counter electromotive force is generated from the rotary drive source.